looking for a circuit....

[jeffdavison]

that would take a tap from the 19 khz pilot tone from an old tube FM multiplex adapter and trigger an led. A diy "stereo" indicator lamp.

Any leads of info would be appreciated.


THanks

Jeff

[EWBrown]

The cheap and easy (or lazy) way is to get an LM1800 or other similar stereo decoder DIP IC and just use the "lock" LED output, and just enough circuitry to make it function. I know, that's "cheating" but also the simple approach


That way, the audio still passes through tubes and only the light uses SS circuitry.

(I'll go into the Dynaco FM-3 diargam and see how they did it with tubes)

THe demod circuit is here:

http://www.geocities.com/vintageaudio/fmx3.JPG

The stereo indicator is part of a "magic eye" tube already in the exsting FM mono tuner, it appears to be driven from the output of the 19KHz to 38KHz pilot tone doubler circuit. That same pilot tone output could be tapped, rectified and the DC signal used to activate a transistor or triode stage which in turn would turn the indicator light on.


FWIW, back in the early days of over-the-air scrambled TV, these LM1800 or LM758 stereo decoder chips and a couple of "one shots" (74123) was the easy method used to re-insert the suppressed horizontal synch and extract the "scrambled" audio (they hid it on the stereo audio channel, and this was in the days before TVs having stereo and SAP as standard functions.

In the Boston area, Channel 68 was the first to use this over the air "scrambling", and I and my froend Rob Carter had it "busted" on the second day of operation . Yellow_Light_Colorz_PDT_01 Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_08 :o Yellow_Light_Colorz_PDT_11

/ed B in NH

[EWBrown]

The basic LM1800 MPX adapter circuit is here on Ned's Tube Zone

http://www.tubezone.net/pdf/diagrams/mpx.pdf

It could be used as a complete mpx adapter, or just use enough of the circuit to light up the "Stereo" signal LED.

/ed B in NH